As the father went outside to get wood for his fire on July 19 last year, he found Johnston lurking near the sleepout and chased him, grabbing him as he tried to scale a fence.
The father put Johnston in an armlock and then applied a choker hold. But when Johnston produced a garden fork and threatened him, the father let go.
A police dog tracked Johnston through the neighbourhood and located him.
The trial was told Johnston had two rape convictions and more than two dozen for burglary.